I wanted our toddler to have some wooden blocks of a size that would let her comfortably build a well-balanced tower, and I had a newish miter saw and a piece of untreated 4x4 left from a babyproofing project.
Make sure your 4x4 is untreated, or treated with something safe for baby to bite on!
Step 1: Cut
I clamped a piece of scrap on my miter saw at just the right distance from the blade to cut the 4x4 and then cut off six blocks. The last cut was close to the blade so I held the wood down with a piece of scrap (the triangle in the second picture).
My 10" Harbor Freight miter saw could almost cut all the 4x4 blocks. The rounded edge of the lumber helped. Some blocks it didn't cut all the way through, leaving a thin segment that got broken off. No problem.
Step 2: Sand
Nothing fancy. Just some sanding to avoid splinters.
Step 3: Play
I was surprised how much easier these large blocks were for the baby to build with.